The expansion of an underground mine (e.g. a coal mine) requires digging a tunnel which initially has an unsupported roof. To stabilize and support the roof a roof bolt must be inserted into the roof to provide support. The operator must first drill holes in the roof through the use of a rotatable cutting bit or roof drill bit. A roof bolt is then inserted into each one of the holes.
A common roof drill bit design uses a cutting insert that has been brazed into a slot at the axially forward end of the roof drill bit body. U.S. Pat. No. 5,400,861 to Sheirer discloses various roof drill bits. U.S. Pat. No. 4,603,751 Erickson also discloses various roof drill bits. Applicants hereby incorporate U.S. Pat. Nos. 4,603,751 and 5,400,861 by reference herein.
While brazed-on cutting inserts have provided adequate results in the drilling of holes, there have been some drawbacks associated with the utilization of the brazed-on cutting inserts. As a result of brazing, the difference in the coefficients of thermal expansion between the steel roof drill bit body and the cemented carbide (e.g., tungsten carbide-cobalt alloy) cutting insert has caused residual stresses in the cemented carbide cutting insert. These residual stresses have been detrimental to the performance of the roof drill bit since they have lead to premature failure of the cutting insert. This has been especially true in those cases where the earth strata being drilled has resulted in high impact loading on the cutting insert.
The presence of these residual stresses also has required that the grades of cemented carbide used for the cutting insert have a high transverse rupture strength. This has been a factor which has limited the number of grades which have been suitable candidates for a cutting insert in a rotatable cutting bit such as a roof drill bit.
Some materials (e.g., ceramics, low binder content tungsten carbide, binderless tungsten carbide, diamond or hard [CVD or PVD] coated ceramics, polycrystalline diamond [PCD] composites with metallic binder (e.g., cobalt) or ceramic binder (e.g., silicon nitride), polycrystalline cubic boron nitride (PcBN) composites) may have been suitable materials for use as a cutting insert in a roof drill bit because of their increased wear resistance, but have been difficult to braze. Other materials such as, for example, hard [CVD or PVD] coated cemented carbides have the increased wear resistance to be a suitable material for use as a cutting insert in a roof drill bit, but the residual brazing stresses have restricted the use of these materials as a cutting insert. As a consequence, these materials have not been realistic candidates for use as cutting inserts in a roof drill bit.
In view of the drawbacks associated with brazing the cutting insert into the slot of a roof drill bit, it would be desirable to provide a roof drill bit wherein the cutting insert would be affixed within the slot of the roof drill bit without using a brazing process. Such a roof drill bit would have less of a chance of premature failure due to the presence of residual stresses. Such a roof drill bit would be able to use a wider range of materials for the cutting insert than has been heretofore available.
There comes a point where the cutting insert in the roof drill bit has reached a condition where the cutting action by the bit is no longer sufficient. At this point one of two processes occurs. One process comprises the regrinding of the cutting insert without removing the cutting insert from the roof drill bit. The other process comprises debrazing the cutting insert so as to be able to remove it from the roof drill bit body, and then brazing a new cutting insert to the roof drill bit body. Each process has certain costs associated therewith which add to the overall cost of the drilling operation.
To reduce these additional costs it would be desirable to provide a roof drilling bit which would not require regrinding to place the cutting insert back in condition for cutting. It would also be desirable to provide a roof drilling bit that does not require debrazing/brazing of the cutting insert to replace a worn cutting insert.